1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio device and a simple path estimating method employed therefor and, more particularly, to a path estimating method employed in a CDMA radio device.
2. Description of the Related Art
Conventionally, path estimation has been performed in receiving operations of a CDMA radio system. FIG. 6A shows the characteristics of a delay profile generated due to multipath of a transmission line in the CDMA radio system. As shown in FIG. 6A, the delay profile consists of magnitude information about electric power, power routes, or the like, and timing information for detecting path timing. In this transmission line, there are three paths of a path (1), a path (2), and a path (3), and lots of noises, notably, large noises (1) and (2).
FIG. 6B shows a delay profile 182, which is obtained by A/D (analog/digital) sampling of a signal in the above transmission line, followed by calculation in a correlator part 81 shown in FIG. 12 according to a conventional method. An A/D sampling point is represented by ◯ in FIG. 6B.
In FIG. 12, a signal 181, which is a digital signal obtained after A/D conversion, is used to correlate it with specific spread codes designated in cell information in the correlator part 81 to create a delay profile 182. An outputted delay profile 182 is used to select a path in a path selection part 82. A path selection method is employed such that the predetermined number of paths are selected in decreasing order of power from the delay profile 182. Alternatively, when such paths have a difference larger than a prescribed value from the maximum path, they may not be determined as a path. The path selection part 82 transfers location information of ultimately determined paths to a spread demodulation part (not shown) as a path timing signal 183.
In the path selection method in the path selection part 82, three largest values of paths are selected in decreasing order of the value from the delay profile 182, at which time up to two points in both sides of the previously detected path are not detected even when they are large values. Furthermore, several conditions are provided such that points having a difference larger than a prescribed value from the maximum path may not be determined as a path.
The path selection part 82 detects the path (1), noise (1), and noise (2) as three paths, and outputs location information thereof as the path timing signal 183. The path (1) and noise (1) and (2) are thus selected by the conventional method.
Another conventional method is such that, in order to reduce a circuit scale for path search to obtain stable search, a signal oversampled by using a sampling signal with a first frequency is re-sampled by using a sampling signal with a second frequency lower than the first frequency at two different timings, and these re-sampling results are added together, followed by de-spreading processing for the added result in a sampling signal cycle of the second frequency, to thereby detect a path appropriate for a receiving operation (see, e.g., patent document 1).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-26765
In a receiving operation of the aforementioned conventional CDMA radio system employing the path estimation, a path may be accidentally synthesized with noise when the noise is misjudged as a path upon the path estimation, which thus degrades receiving characteristics. The same may occur in the method described in the Patent Document 1. Also, particularly in mobile terminals, reduction in power consumption is regarded as an important subject.